Late Devonian extinction

Comparison of the three episodes of extinction in the Late Devonian ("Late D") to other mass extinction events in Earth's history. Plotted is the extinction intensity, calculated from marine genera.

The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennianfaunal stage (the Frasnian-Famennian boundary), about 375–360 million years ago.1citation needed2 Overall, 19% of all families and 50% of all genera went extinct.3 A second, distinct mass extinction, the Hangenberg Event, closed the Devonian period.4citation needed

Although it is clear a massive loss of biodiversity occurred in the Later Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian.5citation needed Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years.6 Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.7

By the late Devonian, the land had been colonized by plants and insects. In the oceans were massive reefs built by corals and stromatoporoids. Euramerica and Gondwana were beginning to converge into what would become Pangaea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the Mesozoic.3 The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested.8 Some statistical analysis suggests the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.95 This might have been caused by invasions of cosmopolitan species, rather than any single event.5 Surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs or other aspects of the Kellwasser event, while agnathans were in decline long before the end of the Frasnian.10citation needed

During the Late Devonian, the continents were arranged differently, with a supercontinent, Gondwana, covering much of the Southern Hemisphere. The continent of Siberia occupied the Northern Hemisphere, while an equatorial continent, Laurussia (formed by the collision of Baltica and Laurentia), was drifting towards Gondwana. The Caledonian mountains were also growing across what is now the Scottish Highlands and Scandinavia, while the Appalachians rose over America; these mountain belts were the equivalent of the Himalaya today.verification needed

The biota was also very different. Plants, which had been on land in forms similar to mosses, liverworts, and lichens since the Ordovician, had just developed roots, seeds, and water transport systems that allowed them to survive away from places that were constantly wet—and consequently built huge forests on the highlands. Several different clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylaleanferns, lepidosigillarioidlycopsids, and aneurophyte and archaeopteridprogymnosperms.13 Fish were also undergoing a huge radiation, and the first tetrapods, such as Tiktaalik, were beginning to evolve leg-like structures.

Duration and timing of the extinction events

Extinction rates appear to be higher than the background rate for an extended period lasting the last 20–25 million years of the Devonian. During this period, about eight to ten distinct events can be seen, of which two stand out as particularly severe.14 The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.15 The fossil record of the following first 15 million years of the Carboniferous is largely void of terrestrial animal fossils, likely related to losses during the end-Devonian Hangenberg event. This period is known as the Romer's gap.1016

The Kellwasser event

The Kellwasser event is the term given to the extinction pulse that occurred near the Frasnian/Famennian boundary. Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record. There may in fact have been two closely spaced events here as shown by the presence of two distinct anoxic shale layers.

The Hangenberg event

The Hangenberg event sits on or just below the Devonian/Carboniferous boundary and marks the last spike in the period of extinction. It is marked by an anoxic black shale layer and an overlying sandstone deposit.17citation needed Unlike the Kellwasser event, the Hangenberg event affected marine and terrestrial habitats.10

Effects of the events

The extinction events are accompanied by widespread oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter. This, combined with the ability of porous reef rocks to hold oil, has led to Devonian rocks being an important source of oil, especially in the USA.

Biological impact

The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and selectively affected shallow warm-water organisms over cool-water organisms. The most important group to be affected by the Kellwasser event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulatecorals. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as oncolites and stromatolites; the reef system collapse was so stark that bigger reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian or "stony" corals) did not recover until the Mesozoic era.

Further taxa to be starkly affected include the brachiopods, trilobites, ammonites, conodonts, and acritarchs. The surviving taxa show morphological trends through the event. Trilobites evolved smaller eyes in the run up to the Kellwasser event, with eye size increasing again afterwards. This suggests vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. The brims are thought to have served a respiratory purpose, and the increasing anoxia of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with δ18O, thus seawater temperature; this may relate to them occupying different trophic levels as nutrient input changed.18 As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.2

The Hangenberg event affected both marine and freshwater communities. This mass extinction affected ammonites and trilobites, as well as jawed vertebrates, including tetrapod ancestors.1019citation needed The Hangenberg is linked to the extinction of 44% of high-level vertebrate clades, including all placoderms and most sarcopterygians, and the complete turnover of the vertebrate biota.10 This led to the establishment of the modern vertebrate fauna, consisting mostly of actinopterygians, chondrichthyans, and tetrapods, in the Carboniferous. Romer's gap, a 15 million-year hiatus in the early Carboniferous tetrapod record, has been linked to this event.10 Also, some of the losses attributed to the Kellwasser event likely actually occurred during the Hangenberg extinction, due to the poor Famennian record for marine invertebrates.1020citation needed

Magnitude

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: a recent survey (McGhee 1996) estimates that 22% of all the 'families' of marine animals (largely invertebrates) were eliminated. The family is a great unit, and to lose so many signifies a deep loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species21 did not survive into the Carboniferous. These latter estimates need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and sampling biases during the Devonian.

Causes of the extinctions

Since the Kellwasser-related "extinctions" occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentological record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate.

Major environmental changes

From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. Evidence exists of widespread anoxia in oceanic bottom waters;13 the rate of carbon burial shot up,13 and benthic organisms were decimated, especially in the tropics, and especially reef communities.13 Good evidence has been found for high-frequency sea-level changes around the Frasnian/Famennian Kellwasser event, with one sea level rise associated with the onset of anoxic deposits.22 The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall;.1723citation needed

Possible triggers

Bolide impact

Bolide impacts can be dramatic triggers of mass extinctions. An asteroid impact was posited as the prime cause of this faunal turnover,242 but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the Kellwasser-aged Alamo and the Hangenberg-aged Woodleigh, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction.1 Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.25citation needed

Plant evolution

During the Devonian, land plants underwent a hugely significant phase of evolution. Their maximum height went from 30 cm at the start of the Devonian, to 30 m26citation needed at the end of the period. This increase in height was made possible by the evolution of advanced vascular systems, which permitted the growth of complex branching and rooting systems.13 In conjunction with this, the development of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas.13 The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian.

Effect on weathering

These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilised a deep layer of soil, which would have been on the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a few centimetres. The mobilisation of a large portion of soil had a huge effect; soil promotes weathering, the chemical breakdown of rocks, releasing ions which act as nutrients to plants and algae.13 The relatively sudden input of nutrients into river water may have caused eutrophication and subsequent anoxia. For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposing organisms use up all available oxygen by decaying them, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatolites and (to a lesser degree) corals—organisms which only thrive in low-nutrient conditions. Therefore, the postulated influx of high levels of nutrients may have caused an extinction, just as phosphate run-off from Australian farmers is causing immeasurable damage to the Great Barrier Reef today.13 Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.25

Effect on CO2

The "greening" of the continents occurred during Devonian time. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced carbon dioxide levels in the atmosphere. Since CO2 is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (located near the Devonian South Pole) suggest widespread glaciation at the end-Devonian, as a broad continental mass covered the polar region. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. The Hangenberg event has also been linked to glaciation in the tropics equivalent to that of the Pleistoceneice age.27citation needed

The weathering of silicate rocks also draws down carbon dioxide from the atmosphere. This acted in concert with the burial of organic matter to decrease atmospheric carbon dioxide concentrations from about 15 to three times present levels. Carbon in the form of plant matter would be produced on prodigious scales, and given the right conditions, could be stored and buried, eventually producing vast coal measures (e.g. in China) which locked the carbon out of the atmosphere and into the lithosphere.28 This reduction in atmospheric CO2 would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall),29citation needed probably fluctuating in intensity alongside the 40ka Milankovic cycle. The continued drawdown of organic carbon eventually pulled the Earth out of its Greenhouse Earth state into the Icehouse that continued throughout the Carboniferous and Permian.

Other suggestions

Other mechanisms put forward to explain the extinctions include tectonic-driven climate change, sea-level change, and oceanic overturning. These have all been discounted because they are unable to explain the duration, selectivity, and periodicity of the extinctions.25